Successfuly upgraded #opnsense to 26. The ISC DHCP migration worked fine, but as it's deprecated and it manages a small LAN, I should eventually migrate to dnsmasq which does DHCP as well apparently.
I have an unbound setting to create DNS records for ISC DHCP clients which seems to be the default in dnsmasq, otherwise it's pretty default.
Just need to figure out how to set fixed IPs/leases for some clients before migrating I guess.